This invention relates to a method of installing a refractory lining in an electric induction furnace, such as a coreless or channel induction furnace, and particularly a method of installing a refractory lining in an electric induction furnace using an electric vibrator.
Electric induction furnaces are used, for example, in the production of molten ferrous and nonferrous metals. These molten metals typically are used to produce castings in foundries from scrap. Induction melting is accomplished by applying an electric current to copper furnace coils, referred to as the primary winding. The current in the primary winding induces a current in the scrap metal within the furnace, referred to as the secondary winding. The current induced in the secondary winding meets electrical resistance and generates heat. When sufficient heat is generated, the scrap metal melts. Induction heating is used not only for melting metals but also for holding metals in the molten state until the metal is removed from the furnace for the production of castings or other processing Application temperatures typically range from about 1000 F. to about 3200 F.
To contain the heat and molten metal within an induction furnace, specialized refractory materials typically are used to line the furnace. Conventional refractory linings for induction furnaces typically are comprised of silica, fused alumina, fused magnesia, calcined magnesia, fused mullite, calcined fireclay, calcined chamotte, calcined bauxite, and zircon refractory aggregate. The refractory lining is a consumable material that is eroded or otherwise damaged by exposure to the conditions within the furnace. Conventional refractory lining materials tend to have a relatively high consumption rate, which corresponds to a short lining life.
When a certain amount of consumption or damage to the lining has occurred, the operation of the induction furnace must be interrupted to repair or replace the refractory lining. The frequency of the interruption is determined by the consumption rate of the refractory lining for a given process. The duration of an interruption depends on the nature and extent of the consumption. When the consumption or damage is extensive, removal and replacement of the entire refractory lining rather than repair of the eroded or damaged portion may be necessary. Replacement of the lining increases furnace downtime. The total furnace downtime depends on the frequency and duration of the interruptions.
The electric induction furnace may be a coreless or a channel furnace. A coreless furnace includes a generally continuous floor portion and walls that extend upwardly from the periphery of the floor portion. A channel furnace includes a floor/throat portion and walls that extend upwardly from the periphery of the floor portion. The floor/throat portion defines an outlet connected to an inductor. The inductor typically includes a metal container or casing that encloses a refractory lining and bushings. The bushings are hollow and typically are made of copper or steel. An induction assembly is enclosed within each bushing, which serves as the primary winding. The refractory lining defines a passageway for molten metal flowing from the outlet. The molten metal is heated further in the inductor.
Refractory linings for the floor portion and walls of electric induction furnaces typically are installed in a two step process. First, the refractory lining is installed in the floor portion of the furnace. Second, the walls of the refractory lining are installed using a liner form that is positioned on the installed floor. The liner form defines the inner wall of the refractory lining. The inner wall of the furnace defines the outer wall of the refractory lining. Channel furnaces require an additional step for installation of refractory lining in the inductor.
The liner form may be removable or consumable. Removable forms typically are used for refractories designed to have low-temperature, heat-set bonds. Removable forms also are desirable to prevent contamination from melting of a consumable form into the molten metal product. Consumable forms typically are used for higher temperature applications (i.e., greater than about 2000 F.) when the melted form can be used as part of the molten metal product. Consumable forms also are used when removal of a form would not be feasible after refractory installation, for example, in the inductor of a channel furnace.
A refractory lining typically is installed in an inductor casing using a solid or hollow loop or channel form and cylindrical or rectangular bushings. The channel form typically is burned or melted away during a heatup process after the refractory has been installed. The decomposition of the channel form leaves a passageway for molten metal inside the refractory cross-section. The refractory is installed into the inductor casing around the channel form and bushings. Generally, the refractory is placed into the bottom of the inductor casing. When a sufficient amount of refractory has been added to the bottom of the casing, the channel form is placed inside the inductor casing. Refractory is then added around the channel form. When a sufficient thickness of refractory has been added around the channel form, at least one bushing is installed. After installation of the bushing(s), refractory is continually installed between the channel form and the inductor casing and around the bushing(s) within the channel form to the top of the inductor casing.
Proper installation of the refractory is essential to prolonging refractory life. A successful installation is judged by the density of the installed refractory materials, i.e., the amount of material installed into an induction furnace for a given volume. Errors during installation that cause the installed refractory lining to have a less than optimal density will reduce the service life of the lining. Optimal density depends on the effective removal of air trapped within the dry refractory material and compaction of the dry refractory particles to reduce the distance between them. Air trapped within the dry refractory material typically is removed by inducing flow in loosely packed dry refractory material.
Refractory installation requires a skilled labor force and can be labor intensive. An installation can take from about three hours to three shifts or more, depending on the size of the furnace.
Conventional methods for installing a refractory lining require the provision of dry refractory material in loose shallow layers. The maximum depth of the layers is about 3-5 inches depending on the type of refractory. For example, silica refractory generally may be installed in layers having a maximum depth of about 5 inches, dry vibratable refractories (including alumina/magnesia/mullite refractories) generally may be installed in layers having a maximum depth of about 4 inches, and chrome-alumina refractories generally may be installed in layers having a maximum depth of about 3 inches. The density of the refractory lining will be reduced if the layers of dry refractory material exceed the maximum depth for that application because it is difficult to properly perform manual deairing of thicker layers.
Manual deairing involves inducing flow in the dry refractory material so that air trapped within the dry refractory material may escape. This typically is accomplished by forking or spading the entire surface of the layer about four times. The density of the installed refractory lining will be reduced if the dry refractory material is not thoroughly deaired. Operator error due to inattention, undue haste or inadequate training may compromise a successful installation. The forking or spading tool typically weighs about 15-20 pounds, which can result in operator fatigue, which also may compromise a successful installation.
Conventional methods of installing refractory linings also require compaction of the dry refractory material using an electric vibrating tamper or form vibration. An electric vibrating tamper typically is used for smaller furnaces. A tamper also may be used for the rapid installation of nonsilica refractory materials in large furnaces (i.e., those with a metal capacity of greater than about 8 tons) because form vibration generally cannot be used effectively and quickly in these applications.
As described above, a shallow layer of dry refractory material is provided in the bottom of the furnace or between a liner form and an inner wall of the furnace and the dry refractory material is manually deaired with a forking or spade tool before being compacted with the tamper. Each layer must be compacted before the next layer of loose refractory material is added. The process is repeated until the refractory lining floor or wall reaches a desired height. An installation typically will involve compaction of numerous layers.
The electric vibrating tamper, such as a Bosch vibrator, is provided with a vibrating foot, with a disk-shaped foot being used for floor installation and a crescent shaped foot being used for wall installation. The electric vibrating tamper is controlled manually by the operator. The amount of force available for compacting the refractory material with an electric vibrating tamper depends upon the centrifugal force produced and the pressure applied by the operator. When operated using a 240-volt power source, rotation of the centrifugal weight of the tamper at 3600 rpm will produce a force of approximately 450 lbs.
Installation of refractory linings using an electric vibrating tamper can be time consuming, ranging from about 3 to 24 hours depending on furnace size. The operator must be skilled and attentive to the installation to avoid premature failure of the refractory lining due to inadequate densification or insufficient knitting of the various refractory layers. This is especially true in the case of channel inductors because of the difficulty in compacting the refractory underneath the channel form and between the channel form and the bushing(s) given the enclosed spaces and the sizes of the deairing tool and vibrating tamper.
Form vibration is most commonly used to install refractory linings in larger furnaces. Form vibration generally is less labor intensive than an electric vibrating tamper because, while the refractory material still must be installed in layers having a maximum depth of about 3-5 inches, with manual deairing of each layer before the addition of a subsequent layer, compaction of the dry refractory material can be carried out in a single operation after all of the dry refractory layers have been added to the liner form. The liner form is backfilled with additional dry refractory material after an initial period of form vibration and then vibration is continued.
Form vibrators typically are powered by compressed air. The availability of a compressed air supply or sufficient quantity and quality is a major disadvantage to the use for form vibrators. Some foundries have difficulty in providing adequate compressed air pressure (80 psi minimum) due to the demands for compressed air by other operations within the foundry. Other foundries have difficulty in providing adequate compressed air volume to the constrictions in air lines. The lack of adequate compressed air pressure or volume may require that the furnace be relined during off-peak hours when air is more readily available. Other foundries have difficulty in providing compressed air of suitable quality. Dirty or wet compressed air will wear the moving components of the vibrator, which comprises vibrator output and result in a less dense refractory lining. This in turn results in a shorter refractory life, causing more frequent relines, increased furnace downtime, and increased operating cost.
Form vibrators may use frequency vibration or impact vibration. Form vibrators that use frequency vibration are generally preferred because the frequency can be measured during operation, e.g., using a Vibra-Tak vibration indicator. The frequency measurement allows the operator to determine if the vibrator is operating properly and to calculate the approximate vibrational force being applied. Conventional form vibrators that use frequency vibration may be low amplitude, high frequency vibrators or high amplitude, low frequency vibrators. Low frequency, high amplitude vibrators, such as the Martin BRUTE, typically operate between about 2500 and 4000 rpm. The high amplitude vibration is particularly effective for inducing dry material to flow. Use of low frequency, high amplitude vibrators is limited to lower density refractories (e.g., densities in the range of 130 lb./ft3). High amplitude vibrators also are generally not suitable for use in small furnaces due to the high force exerted by the vibrator during use. Multiple vibrator locations, typically eight to twelve locations depending on furnace size) are required for effective installation.
High frequency, low amplitude (mass) vibrators, such as the Martin VIBROTOR, typically operate between about 4800 and 6500 rpm. The low amplitude vibration is particularly effective for compacting the dry refractory material. The use of high frequency, low amplitude vibrators has been found to be effective using multiple refractory systems in the 130-200 lb./ft3 density ranges. The number of vibrator locations typically is lower than for low frequency, high amplitude vibrators.
Conventional vibrators typically are used by bolting the vibrator to an iron frame that has been welded to the interior of the form or the furnace structure. Other methods for causing the vibrator to engage the form also may be used. For example, in the expanding Netter-Cross vibrator, a hydraulic pump forces oak runners out into contact with the form and then a pneumatic vibrator shakes the form to compact the dry refractory material.
Form vibrators that use impact vibration have numerous pistons that strike the inside of the form. One commonly used impact vibrator has three pneumatically powered whipping hammers or jackhammers extending radially from a cylinder, which is suspended from a crane over the liner form The cylinder rotates on a carousel relative to the form such that the hammers can engage the entire circumference of a generally circular liner form.
Impact vibrators have several disadvantages. Because impact vibrators are not harmonic vibrators, the vibration frequency cannot be measured in the field, so it is difficult for an operator to determine if the vibrator is operating properly and sufficient compaction has been achieved. The vibrator must be moved to multiple locations within the liner form to ensure that the vibrator has effectively impacted the entire volume of the form. A skilled, experienced operator is needed to determine the locations where the vibrator is to be positioned. The intense forces associated with impact vibration also tend to limit the service life of the vibrator. For example, the three-hammer vibrator described above tends to have a service life of about one year compared to about 3-5 years for harmonic form vibrators.
It is an object of the invention to provide a method of installing a refractory lining for an electric induction furnace that avoids the need to add dry refractory to the lining walls in layers with manual deairing of each layer before addition of the next layer.
It also is an object of the invention to provide a method of rapidly installing a refractory lining for an electric induction furnace that reduces downtime and operator error during installation and replacement of the lining.
It is another object of the invention to provide a method of rapidly installing a more consistent refractory lining for an electric induction furnace using a variable frequency/amplitude electric vibrator.
It is still another object of the invention to provide a method of installing a refractory lining that avoids the disadvantages associated with conventional installation methods using vibrators powered by compressed air.
It is yet another object of the invention to provide a method of rapidly installing nonsilica refractory linings in large furnaces (i.e., those having a metal capacity of greater than about 8 tons) without using an electric vibrating tamper.
It is yet another object of the invention to provide a method of rapidly installing silica and nonsilica refractory linings in channel inductors without using an electric vibrating tamper or manually deairing the refractory.
These and other objects of the present invention will be apparent from the specification that follows, the appended claims, and the drawings.
The present invention overcomes the problems associated with conventional refractory lining methods. The method uses a variable frequency/amplitude electric vibrator. The electric vibrator provides a precise output, which results in a repeatable process that is not dependent on adequate air pressure or quality or operator skill. The repeatability of the process results in more consistent lining installations, which corresponds to longer lining service life. The method allows elimination of the layering of the dry refractory material in the walls and the manual deairing of each layer. This reduces furnace downtime and labor costs associated with installation and replacement of refractory linings.
The present invention provides a method of installing a refractory lining in a wall of an electric induction furnace, the method comprising the steps of providing a dry refractory material between a liner form placed within the furnace and a furnace wall, placing at least one variable frequency/amplitude electric vibrator in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one variable frequency/amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration, and compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode having low amplitude, high frequency vibration after inducing flow of the dry refractory material. The method also may include the step of providing additional dry refractory material between the liner form and the furnace wall before compacting the dry refractory material.
Preferably, the variable frequency/amplitude vibrator has a rotating shaft. The method may include the steps of causing the shaft to rotate in a first rotational direction in the first operational mode and in a second rotational direction in the second operational mode and selecting a shaft rotational axis to maximize refractory densification.
The method also may include the steps of providing a programmable controller in operational communication with at least one variable frequency/amplitude vibrator, the controller being capable of storing and sending instructions for direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation and storing instructions for preselected operational modes, with each operational mode defining a direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation. The method may further include the step of causing the programmable controller to store instructions for at least one preselected operational mode. In another embodiment of the invention, the method of installing a refractory lining in a wall of an electric induction furnace may include the steps of providing a dry refractory material between a liner form placed within the furnace and a furnace wall, placing at least one variable frequency/amplitude electric vibrator in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, operating at least one variable frequency/amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration such that flow of the dry refractory material is induced and the volume of the dry refractory material is reduced, and operating at least one variable frequency/amplitude vibrator in a second operational mode having a low amplitude, high frequency vibration such that compaction of the dry refractory material occurs after flow of the dry refractory material is induced. The method further may include the step of providing additional dry refractory material between the liner form and the furnace wall before compacting the dry refractory material.
In still another embodiment of the invention, the method of installing a refractory lining in a wall of an electric induction furnace consists essentially of the steps of providing a dry refractory material between a liner form placed within the furnace and a furnace wall, placing at least one variable frequency/amplitude electric vibrator in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one variable frequency/amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration, and compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode having a low amplitude, high frequency vibration after inducing flow of the dry refractory material.
In yet another embodiment of the invention, the method of installing a refractory lining in a wall of an electric induction furnace may include the steps of providing a dry refractory material between a liner form placed within the furnace and a furnace wall, placing at least one variable frequency/amplitude electric vibrator having a rotating shaft in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one variable frequency/amplitude vibrator in a first operational mode having a low frequency vibration and a first shaft rotational direction, and compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode after inducing flow of the dry refractory material, the second operational mode having a high frequency vibration and a second shaft rotational direction, the said first shaft rotational direction causing the vibrator to vibrate at high amplitude and the second shaft rotational direction causing the vibrator to operate at low amplitude. The method further may include the step of adding additional dry refractory material between the liner form and the furnace wall after the volume of the dry refractory material has been reduced. The method also may include the step of selecting a shaft rotational axis to maximize refractory densification.
The method also may include the steps of providing a programmable controller in operational communication with at least one variable frequency/amplitude vibrator, the controller being capable of storing and sending instructions for direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation and storing instructions for preselected operational modes, each operational mode defining a direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation. The method may further include the step of causing the programmable controller to store instructions for at least one preselected operational mode.
In another embodiment of the invention, a method of installing a refractory lining in a wall of an electric induction furnace includes the steps of providing a first quantity of a dry refractory material between a liner form placed within the furnace and the furnace wall, placing at least one variable frequency/amplitude electric vibrator having a rotating shaft in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one variable frequency/amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration in the first operational mode, compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode having a low amplitude, high frequency vibration after inducing flow of the dry refractory material, and providing a programmable controller in operational communication with at least one variable frequency/amplitude vibrator, the controller being capable of storing and sending instructions for direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation, a first direction of vibrator shaft rotation causing operation of the vibrator at high amplitude and a second direction of vibrator shaft rotation causing operation of the vibrator at low amplitude. The programmable controller may be capable of storing instructions for preselected operational modes, each operational mode defining a direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation, and the method may further include the step of causing the programmable controller to store instructions for at least one preselected operational mode.
The present invention also includes a method of installing a refractory lining in the floor portion of an electric induction furnace, the method including the steps of providing a dry refractory material in the floor portion of the furnace, providing at least one variable frequency/amplitude electric vibrator equipped with a vibrating bottom plate, and compacting the dry refractory material by causing the vibrating bottom plate to engage the dry refectory material when the attached variable frequency/amplitude vibrator is in an operational mode having a low amplitude, high frequency vibration after inducing flow of the dry refractory material. The method may further include the step of inducing flow of the dry refractory material before the compacting step by causing the vibrating bottom plate to engage the dry refractory material when the variable frequency/ amplitude vibrator is in an operational mode having a high amplitude, low frequency vibration. The method also may include the step of preparing the installed refractory lining floor portion for wall installation by scratching the surface of the periphery of the floor portion.
In the above-described floor installation method, at least one variable frequency/ amplitude vibrator may have a rotating shaft and the method may further include the step of causing the shaft to rotate in a first rotational direction in the first operational mode and in a second rotational direction in the second operational mode. The method also may include the step of providing a programmable controller in operational communication with at least one variable frequency/amplitude vibrator, the controller being capable of storing and sending instructions for direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation and storing instructions for preselected operational modes, with each operational mode defining a direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation. The method further may include the step of causing the programmable controller to store instructions for at least one preselected operational mode.
The invention also provides a method of installing a refractory lining in an electric induction furnace, including the steps of installing a refractory lining in the floor portion of the furnace, providing a dry refractory material between a liner form placed on the floor portion of the refractory lining and the furnace wall, placing at least one variable frequency/amplitude electric vibrator in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one variable frequency/amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration, and compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode having a low amplitude, high frequency vibration after inducing flow of the dry refractory material. The method may further include the step of adding additional dry refractory material between the liner form and the furnace wall before compacting the dry refractory material.
In the above described method, at least one variable frequency/amplitude vibrator may have a rotating shaft and the method may comprise the steps of causing the shaft to rotate in a first rotational direction in the first operational mode and in a second rotational direction in the second operational mode and selecting a shaft rotational axis to maximize refractory densification.
The method also may include the step of providing a programmable controller in operational communication with at least one variable frequency/amplitude vibrator, the controller being capable of storing and sending instructions for direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation and storing instructions for preselected operational modes, each operational mode defining a direction of vibrator shaft rotation, vibrator speed, and time of vibrator operation. The method further may comprise the step of causing the programmable controller to store instructions for at least one preselected operational mode.
In the above-described method, the step of installing a refractory lining in the floor portion of the furnace may comprise the steps of providing a dry refractory material in the floor portion of the furnace, attaching a vibrating bottom plate to at least one variable frequency/amplitude electric vibrator, and compacting the dry refractory material in the furnace floor portion by causing at least one vibrating bottom plate to engage the dry refectory material in the furnace floor portion when the attached variable frequency/amplitude vibrator is in an operational mode having a low amplitude, high frequency vibration. The floor installing step may further include the step of inducing flow of the dry refractory material in the furnace floor portion before the compacting step by causing at least one vibrating bottom plate to engage the dry refectory material in the furnace floor portion when the attached variable frequency/amplitude vibrator in an operational mode having a high amplitude, low frequency vibration. The floor installing also may include the step of preparing the installed refractory lining floor portion for wall installation by scratching the periphery of the surface of the floor portion.
The invention also includes a method of installing a refractory lining in an inductor of an electric channel induction furnace, including the steps of placing a channel form inside an inductor casing, installing at least one bushing spaced at a distance from the interior of the channel form, filling the inductor with dry refractory material, placing at least one variable frequency/amplitude electric vibrator in operational communication with an element selected from the inductor casing and the channel form, inducing flow of the dry refractory material by operating at least one variable frequency/ amplitude vibrator in a first operational mode having a high amplitude, low frequency vibration, and compacting the dry refractory material by operating at least one variable frequency/amplitude vibrator in a second operational mode having low amplitude, high frequency vibration after inducing flow of the dry refractory material.
In another preferred embodiment of the invention, a method of installing a refractory lining in a wall of an electric induction furnace includes the steps of providing a dry refractory material between a liner form placed within the furnace and a furnace wall, placing at least one electric vibrator in operational communication with an element selected from a structural member of the furnace, the liner form, and a vibrator rig placed within the furnace, inducing flow of the dry refractory material by operating at least one electric vibrator in a first operational mode having a high amplitude, low frequency vibration, and compacting the dry refractory material by operating at least one electric vibrator in a second operational mode having low amplitude, high frequency vibration after inducing flow of the dry refractory material.
Additional features and advantages of various preferred embodiments of the invention will be better understood in view of the detailed description provided below.